Project One has two focus areas. In the first focus area we shall use in vivo rat models of mammary carcinogenesis to define the role of specific environmental agents on the following: 1. Morphological mammary gland development, 2. Genetic and biochemical expression patterns of mammary gland development, 3. Dose and age-dependent criteria of carcinogen susceptibility. Windows of susceptibility for mammary gland carcinogenesis are carcinogen, dose, and age dependent. Therefore, we have chosen as our model carcinogens two agents that are completely different with regard to chemistry, mechanism of initiation, and target window of mammary gland sensitivity: irradiation and benzo[a]pyrene (B[a]P). The hypothesis for this focus area is that fatty acids (FA) and phytoestrogens, through modification of estrogen synthesis, metabolism, and signaling, define not only mammary gland rates of maturation, but also the physiology of the animal in ways that impact the time frame of mammary gland susceptibility to initiation. In Specific Aim 1 we shall examine the potential for FA to alter degree and/or the age range in which the mammary gland is susceptible to initiation. Life-long diet composition will include high levels of n-6 PUFA, n-3 PUFA, or monounsaturated FA on a human low fat equivalent diet (20%) and high fat diet (40%), the latter including both lean and obese cohorts. Specific Aim 2 focuses on phytoestrogens. Isoflavones from soy protein or flaxseed extract will be introduced into the most carcinogen-stimulating diet identified in Aim 1. With each diet we shall examine degree and age of susceptibility to irradiation and B[a]P, with initiation at birth, weaning, 45 days, and 120 days, defining biochemical, morphological, and gene expression data that discriminate between mammary cells pre and post-initiation. The second focus area, Specific Aim 3, addresses the development of a mechanism-based screen for mammary carcinogens. Through analysis of isolated mammary epithelium initiated in vitro by four mechanistically unique mammary carcinogens (irradiation, B[a]P, 4-aminobiphenyl, and ochratoxin A) and four carcinogens that do not cause mammary cancers in animals (dibenz(c,g)carbazole, aminofluorene, d-limonene, and safrole), we shall define the biochemical, metabolic, and gene expression data that provide a fingerprint for an initiated mammary epithelial cell.